TiO2 thin films were obtained on unheated glass substrates by a DC reactive magnetron sputtering method. The as-deposited films exhibit an amorphous structure as observed from X-ray diffraction (XRD) patterns. The structure changes to a mixed one of 70% anatase and 30% rutile after heat treatment in air in the temperature range 293-673 K. Using ellipsometric measurements, and a computer to solve the corresponding equations, a modeling technique was used to find the optical constants of the studied thin films. A sensitivity analysis was performed.